The invention relates to a switching arrangement for telecommunication switching systems, more particularly telephone switching systems. The invention is particularly directed to apparatus wherein the idle condition of switching elements ready to be seized by the traffic carried is marked at a test point by a potential applied to the test point over resistances of a seizing circuit of the switching element involved, and the blocked condition is indicated by the absence of that potential, or vice versa.
This type of switching arrangements has become known in the art, for example, through West German Auslegeschrift No. 1 940 847. A widely known technique in automatic telephone systems is to determine through test relays the busy/idle condition of seizing wires connected to switches, repeaters and similar switching facilities. There test relays first test the switching device by connecting it to a comparatively high impedance and, if it is in the idle condition, seize and guard it against seizure by connecting it to a low impedance. In the idle condition the tested switching device offers a potential, e.g., a negative potential, that indicates this condition via its seizing circuit at a test point allocated thereto.
A testing switch operates from its test circuit with a reverse potential, e.g., a positive potential (ground potential). When a switch finds an idle output, i.e., when a test circuit of a testing switch is connected to a seizing circuit of a following idle device, whose test point carries the potential indicating the idle condition, the test relay of the switch operates. It connects the reverse potential to the test point of the seizing circuit of the device so seized reducing the resistance of the circuit, that is, it rejects the potential indicating the idle condition. The test relay of the switch is held via the thusly closed circuit. Other switches tested thereafter find almost the reverse potential (strictly speaking, a partial voltage) on the seizing circuit, thus recognizing the relevant device as busy.
Consequently, when a busy device is being tested, there exists a branched circuit. The common part of this divided circuit comprises the seizing circuit of the busy device. One of the two branches of the divided circuit comprises a rejection filter of the switch that has previously seized the succeeding device. The low impedance winding of the test relay of this switch is disposed in this branch. The other of the two branches is the test circuit of the other switch which finds the following device inhibited (busy). In this branch the low impedance and the high impedance windings of the test relay of the second switch are connected in series. The branching point is the test point of the seizing circuit of the following device.
Because the rejection circuit of the first switch is of substantially lower impedance than the test circuit of the second switch, one obtains a branch circuit at the common test point, the splitting of the total current in the seizing circuit of the following device into two partial currents being such that by far the predominant part of the total current flows through the rejection circuit and the test relay of the first switch which thus was the first to attempt to seize the succeeding device. The partial current in the test circuit of the second switch and its two test relay windings is so weak as to make the test relay unresponsive. Thus, it is ensured that the test relay of the first switch remains operated and the test relay of the second switch cannot operate.
A more difficult situation develops in the special case where two switches are tested in parallel. In this case, the two circuits of two switches tested independently of one another are randomly and simultaneously connected to the test point of a following device. Assuming that the test relays of both switches have the same resistances, which may reasonably be expected, the current is subdivided in the ratio of 1:1, i.e., half the current in the seizing circuit of the following device flows in the windings of the test relays of both switches. In this case, both test relays must not operate. This condition is hard to comply with. Therefore, the low impedance guarding by the test relay which is the first to operate its contacts prevents the other test relay from operating also.
To improve the above conditions for preventing dual testing when two switches are tested in parallel, two test relays per switch have been provided in known switching arrangements (e.g., West German Pat. Nos. 1 013 701 and 1 165 678), of which a first high-speed test relay for stopping the switch energizes a second test relay (auxiliary test relay). The second test relay serves to improve the certainty of preventing the dual testing of two switches being tested in parallel.
West German Auslegeschrift No. 1 940 847 discloses a technique for increasing the resistance of seizing circuits after seizure. The same publication also discloses a technique for connecting reverse potential to its own seizing circuit over its own resistor, not only from a testing and guarding switch, but also inside a device seized by a preceding switch. This principle is reduced to practice, among other things, through the use of an opposing winding of the seizing relay which is provided in lieu of the resistor mentioned above. This opposed winding is so designed that the seizing relay receives adequate holding energization even after it has been actuated. As the preceding switch is released, the current in the opposed winding increases so that the seizing relay is rejected through opposing energization. Energization of the resistor or the opposing winding decreases the current in the test circuit of the guard switch, but not in the seizing circuit. The potential at the test point of the device involved is moved toward the reverse potential.
The increase in resistance referred to hereinabove and effected by looping an additional resistor into the seizing circuit and the subsequently described connection of reverse potential inside a device seized by a preceding switch via an additional resistor or via an opposed winding of the seizing relay are designed both to increase the reliability of guarding inhibited seizing circuits and to prevent a seizure attempt on releasing connections. A seizure attempt here refers to an operation wherein as a preceding switch is released, prior to the release of the seizing relay of the switching device seized until then and guarded by it and becoming idle as a result of the release, another switch tests or seeks to seize the switching device before the seizing relay has released. Due to partial energization on a previous step of the switch the test relay of a switch, owing to a parallel process or the like, is capable of operating fully on the next step even on a potential normally inadequate for seizure attempts. However, by increasing the resistance, this effect can be prevented by the use of the test circuits described in West German Pat. Nos. 1 013 701 and 1 165 678.
These aforementioned test circuits require one test relay and one auxiliary test relay per switch. In the case of switches having only a single test relay, i.e., not provided with a test circuit as disclosed in West German Pat. Nos. 1 013 701 and 1 165 678, seizure attempts cannot be prevented by connecting, as described hereinabove, the reverse potentials inside a device seized by a preceding switch to its own seizing circuit of the seized device over its own resistor or over an opposed winding of the seizing relay.
Accordingly, it is an object of the invention to guard seizing circuits of switching devices ready to be seized by preceding switches or the like, not only against the hazards of dual testing when two switches are tested in parallel, but also against the hazards of seizure or reseizure attempts.